ISEMPTY()
1 if STACK: TOP == 0
2 return TRUE
3 else return FALSE

PUSH(ELEMENT)
1 STACK.TOP = STACK.TOP + 1
2 STACK[STACK.TOP] = ELEMENT

POP()
1 if ISEMPTY() == TRUE
2 ERROR “underflow”
3 else STACK.TOP = STACK.TOP - 1
4 return STACK[STACK.TOP + 1]

PEEK()
1 return STACK[STACK.TOP]

SIZE()
1 return STACK.TOP